In my experience it was the worst visa run I did.last December, got it by train, getting in was OK, but the land border to get back in Thailand was incredibly crowded, smashed in room for more than an hour with no ac and no fan. It was the land border you cross by foot or vehicle, I suppose the one you do by train my be better..(I didn't want to wait until my next train.. Terrible idea..) But I suppose if you are already in hat yai, being so close to you, is still your better and cheapest option.